1. Field of the Invention
The present invention relates to substantially kinematic steering of a vessel in a yaw path having a given radius of curvature conforms with a predetermined control law.
2. Description of the Prior Art
The navigation and steering problems which will be dealt with pertain to vessels which have a steering-dynamic inertia such that water-ways intended for such vessels can be regarded as narrow. Examples thereof are large tankers when mooring along a coast or large ferries maneuvering in coast traffic. In the following, the actual water-ways for such vessels will be referred to as navigable passages. The fundamental principle of navigation in such passages is to check continuously the position of the vessel. The navigator knows where the vessel is relative to suitable reference marks (beacons, lights, capes, etc.). The position of the vessel expressed in absolute coordinates, for instance longitude and latitude, is not essential. However, the position is checked by observations, for instance ocular observations, and radar observations of the surroundings. In navigable passages, courses are taken from charts or by means of course lines on a navigation radar indicator. An established course is maintained by observing the compass and by observing the yaw angular rate on an instrument.
Small course changes do not create any major problems. The need for such changes can be recognized by simple ocular observations e.g., when navigating along leading lines, or by a course line on a radar indicator. Also, the steering onto such corrected courses does not seem to a dominating problem. The yaw angle rate instrument seems to be a good facility for such steering. In connection with large course changes where the narrowness of the yaw is dependent on passage conditions, the difficulties in steering will increase rapidly. It is difficult to predict as accurately as desired the position after the yaw has been completed. A common method of steering during a yaw can be described as follows.
The vessel moves along a straight course in the passage, here termed the actual course. It is required to turn through a gate and after said gate a new straight course has to be obtained, here termed the exit course. The exit course can be taken from a chart or by means of a course line on the radar indicator. The yaw is carried out by ordering a certain rudder deflection (rudder angle) which is based on experience.
Thus, the yaw carried out as described above is initiated at a certain "set point" which can be determined by observations (visually or by radar). Due to inertia of the rudder system, the moement of the vessel, etc., the set point is selected to lie a certain distance, determined by experience, ahead of the point of the path where the yaw is assumed to have a more or less fixed yaw center. Considering the desired narrowness of the yaw, the commander orders a certain rudder deflection, for instance starboard 20.degree.. In view of the decrease of speed due to the yaw, drift, etc., the commander cannot be quite sure of the position of the vessel after the yaw has been completed. Relatively large position errors can occur relative to the navigable passage. There is a need for experienced and observant navigators and observers in order to achieve a good yaw.
Just before the new straight course (i.e., the exit course) has been obtained, the commander orders (repeats) the new course, and the helmsman abandons the fixed rudder angle and, by means of a compass, the yaw angle rate meter and general experience, he steers to the new course. During this transition, corrections must also be made for errors in the vessel's lateral position which occurred during the yaw. To effectuate such corrections, maneuvering space is needed. Such space is obtained by making the yaw narrower than would otherwise have been necessary in view of the bottom topography or other obstacles. Narrow yaws in navigable passages are quite burdensome, both as to the personnel involved as well as economically. The personnel must have highly developed powers of observation and a great amount of experience, both being of great importance for the safety of the steering procedure. Economically, a narrow yaw represents a loss of energy due to a decrease in speed. The speed can decrease considerably during a narrow yaw. The comfort of the passengers can also be adversely affected by a narrow yaw.
A proven alternative way of steering is to predict the movement and the path of the vessel for a number of vessel lengths ahead by means of instrument calculation facilities, for instance a computer, with due consideration given to the external environment (bottom characteristics, drift, etc.), the rudder deflection of the vessel, state of movement and the dynamic characteristics of the vessel. The predicted path is presented, e.g., on a radar indicator and thus can form the basis for steering. The drawback of such a method is that well defined information about the dynamic characteristics of the vessel is required and such characteristics often change very complicated (involving up to several tens of dynamic parameters). Since the parameters can change due to, for instance the bottom characteristics in shallow waters, the overall accuracy of steering can be poor. The foregoing indicates that there exists a need and a desire for an improved method of navigation and steering.